Following kranazoli’s suggestion - thanks - about the Add-on Compatibility Reporter, WebRep is now working well, as far as I can tell, in the Firefox 5 beta I have.

Regarding DavidR’s opinion about “chasing” betas, we simply disagree. I see an add-on update as being a trivial matter, usually anyway, and beta users have been accommodated quickly by thousands of other add-on developers.